The chemokine receptors CCR5 and CXCR4, which are members of the GPCR family of membrane proteins, mediate the entry of HIV. In this application we propose to develop a new method for NMR-based drug discovery and optimization targeting CCR5 embedded in virus like particles (VLP). The rationale behind our studies is that CCR5 are enriched in membrane lipid rafts, which is also the site of VLP formation and budding, and that the VLP will provide a native membrane consisting of lipids and cholesterol. Accordingly, we hypothesize that VLP produced in cell lines overexpressing CCR5 will be valuable model membrane system for biophysical and drug discovery studies. With respect to whole cell systems, VLP present numerous advantages including: an inert and non-metabolizing system, the ability to purify and concentrate, and the absence of contaminating intracellular components. The specific aims are: (1) Antagonists of CCR5 will be discovered by NMR-based screening of a fragment library; (2) NMR-guided chemical optimization of the most promising fragments will be initiated. Together these studies present a first step toward the development of novel anti-HIV agents targeting CCR5 in the near term and CXCR4 in the long term. To our knowledge, the VLP-based approach toward drug discovery and optimization has not been pursued before and thus this application may be considered High Risk. Nonetheless, the methodologies under development are expected to apply to GPCRs in general, which are the target of ~40% of all drugs, and thus this application presents the potential to have a very significant impact for human health. Together, we feel that the High Risk and High Reward aspects of the proposed research render it ideally suited for the R21 mechanism of support.